We have conducted two studies on the role of cellular and humoral immune responses against HBV and HCV in health care workers. Project 1: HBV infection can be prevented by vaccination with HBV surface (HBs) antigen, which induces HBs-specific antibodies and T cells. However, the duration of vaccine-induced protective immunity is poorly defined for healthcare workers who were vaccinated as adults. Here, we investigated the immune mechanisms (antibody and T cell responses) of long-term protection by the HBV vaccine in 90 healthcare workers with occupational exposure to HBV, 10-28 years after vaccination. We found that 59 of 90 health-care workers (65%) had levels of antibodies against HBs (anti-HBs) above the cut-off (>12 mIU/ml) and 30/90 (33%) had HBs-specific T cells that produced interferon IFN-gamma. Anti-HBs titers correlated with numbers of HBs-specific IFN-gamma-producing T cells, but not with time after vaccination. Whereas occupational exposure to HBV after vaccination did not induce antibodies to the HBV core protein (HBcore), the standard biomarker for HBV infection, CD4+ and CD8+ T cells against HBcore and polymerase antigens were detected. Similar numbers of HBcore- and polymerase-specific CD4+ and CD8+ T cells were detected in health-care workers with occupational exposure to HBV and in patients who acquired immunity via HBV infection. Most of the HBcore- and polymerase-specific T cells were CD45RO+CCR7-CD127- effector memory cells in exposed health-care workers and in patients with acquired immunity. In contrast, most of the vaccine-induced HBs-specific T cell cells were CD45RO-CCR7-CD127- and terminally differentiated. In conclusion, HBsAg vaccine-induced immunity protects against future infection but does not provide sterilizing immunity, as evidenced by HBcore- and polymerase-specific CD8+ T cells in vaccinated health care workers with occupational exposure to HBV. The presence of HBcore- and HBV polymerase-specific T-cell responses is a more sensitive indicator of HBV exposure than detection of HBcore-specific antibodies. Project 2: Some of the largest studies on virus exposure in health care workers were conducted by the Centers for Disease Control and Prevention Needlestick Surveillance Group as multicenter case/control studies. In those studies, health care workers who developed antibodies to either HCV or HIV were classified as cases, whereas those who did not seroconvert were classified as controls. Likewise, a large surveillance study in Europe followed 245 health care workers for 5 years using seroconversion as evidence of exposure. However, none of these prospective studies assessed HCV-specific T cell responses. T cell responses are relevant because the small percentage, approximately 20%, of patients who clear acute HCV infection spontaneously mount vigorous HCV-specific T cell responses, and T-cell-mediated immune memory may protect upon re-exposure. HCV-specific T cell responses were also described in cross-sectional studies on seronegative subjects such as injection drug users, family members of HCV-infected patients and health care workers, who test negative for HCV RNA despite an increased risk of HCV exposure. Based on these studies it has often been suggested that years of low-dose HCV exposure may prime and maintain HCV-specific T cells that protect against systemic infection. However, the cross-sectional design of those studies left unclear whether T cells were indeed induced by low-level HCV exposure without seroconversion, or whether they resulted from regular acute infection with subsequent antibody loss. Here, we prospectively studied 72 health-care workers for viremia, antibody and T cell responses longitudinally after documented exposure to HCV via accidental needlestick or cutaneous cut. We found that all health-care workers remained negative for HCV RNA and antibodies, but 48% developed proliferative T cell and 42% IFN-gamma ELISPOT responses, with 29 healthy, HCV unexposed controls used to define assay cut-offs. The response prevalence was associated with the transmission risk score. T cell responses peaked at week 4, and returned to baseline by week 12 after exposure. They predominantly targeted nonstructural HCV proteins, which are not part of the HCV particle and thus, must have been synthesized in infected cells. In conclusion, subclinical transmission of HCV occurs frequently, resulting in infection and synthesis of nonstructural proteins despite undetectable systemic viremia. T cell responses are more sensitive indicators of low-level HCV exposure than antibodies.